Re: [Qemu-devel] tcg: How 'CPUState::current_tb' is used?

[Sergey Fedorov <serge.fdrv@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 1770 bytes --]

On 02/05/16 22:54, Sergey Fedorov wrote:
> Hi,
>
> I can't figure out how this field is used. The comment says it's
> "Currently executing TB", but actually it's the first TB in a chain of
> TBs executed. Grep shows the only place it is really checked is
> tb_invalidate_phys_page_range(). That code seems to be introduced long
> ago in:
>
>     commit ea1c18022edd0e2c45552d6fc2da6e15a3486b33
>     Author: bellard <bellard@c046a42c-6fe2-441c-8c8c-71466251a162>
>     Date:   Mon Jun 14 18:56:36 2004 +0000
>
>         fixed self modifying code in case of asynchronous interrupt
>
>
> I suspect it's only related to user emulation. But I would appreciate
> if someone could give me an idea of how this really works :)

UPD: 'CPUState::current_tb' was used in that version of QEMU by this code:

    /* mask must never be zero, except for A20 change call */
    void cpu_interrupt(CPUState *env, int mask)
    {              
        TranslationBlock *tb;
        static int interrupt_lock;
                             
        env->interrupt_request |= mask;
        /* if the cpu is currently executing code, we must unlink it and
           all the potentially executing TB */
        tb = env->current_tb;
        if (tb && !testandset(&interrupt_lock)) {
            env->current_tb = NULL;
            tb_reset_jump_recursive(tb);
            interrupt_lock = 0;
        }
    }


cpu_interrupt() has changed almost completely since that time. I'm
wondering if checking 'cpu->current_tb' by this code in
tb_invalidate_phys_page_range() still makes any sense:

    if (cpu->interrupt_request && cpu->current_tb) {
        cpu_interrupt(cpu, cpu->interrupt_request);
    }


BTW, I'm not sure about the purpose of this piece of code either :)

Kind regards,
Sergey

[-- Attachment #2: Type: text/html, Size: 2723 bytes --]